Fly-by-wire (“FBW”) aircraft flight control systems are increasingly becoming the preferred type of flight control system for modern aircraft. The FBW type of control system replaces the relatively heavier and more error prone mechanical and hydro-mechanical types of flight control systems.
Stated simply, a fly-by-wire aircraft flight control system comprises a computer system interposed between the flight control inputs given manually by the pilot and co-pilot (and automatically by various aircraft subsystems) and the flight control surfaces that ultimately control the direction of the aircraft in flight. That is, the inputs from the pilot or co-pilot are not connected directly to the aircraft flight control surfaces desired to be controlled (e.g., ailerons, rudder, elevators, spoilers, slats, flaps, etc.). Instead, the pilot and co-pilot inputs are routed to a computer system (e.g., typically comprising more than one computer or data processor type of device) that contains the flight control logic which interprets the pilots' inputs and moves the aircraft flight control surfaces according to control laws (“CLAWS”) stored in the computer system to effect changes in the aircraft's pitch, roll, yaw, altitude, etc., for example. In the alternative, the computer system can be replaced partly or entirely with analog electronic circuits to achieve the same result. However, the clear trend is to use digital computers that contain the flight control logic and which are interposed between the primary pilot control input devices (e.g., sidestick or yoke, rudder pedals) and the actuators associated with their respective aircraft flight control surfaces.
FBW control systems represent a relatively large weight savings (and, thus, significantly reduced fuel costs) on the aircraft as compared to the traditional mechanical or hydro-mechanical flight control systems as the relatively heavy and bulky cables and associated mechanical components of the traditional systems are replaced by wires and relatively simple actuators. Other advantages of FBW systems include a reduction in the workload of the pilots, reduced maintenance time and costs, and increased flight safety as the flight control laws and overall flight envelope can be more precisely tailored to the pilots' sidestick or yoke control input devices. The FBW control system even allows for “automatic pilot” operation of the aircraft in certain flight situations as the flight control computer is typically responsive to various sensor inputs and directs the aircraft flight control surfaces according to the control laws—all without pilot input or involvement.
However, FBW control systems are not without their drawbacks. The older mechanical and hydro-mechanical flight control systems tended to fail gradually over time. This made it relatively easy to identify and correct in advance for any such failures. In contrast, the computer-based FBW control systems tend to fail “completely” in that the computer system running the flight control laws may suddenly “crash” and leave the pilots without the ability to control the aircraft. Thus, typically some type of redundancy is built into a FBW system. For example, three or four computers may be used that are redundantly connected (e.g., in a “triplex” or “quadruplex” configuration) and may even be of different hardware and/or software design to avoid a multiplicity of computer failures at any one time. That way if one of the flight control computers fails then two or three other flight control computers are still operational and can control the aircraft. A FBW system may even have a mechanical flight control system as a backup in case of a failure of the flight control computer(s).
In a FBW flight control system, the primary input device for the pilot and co-pilot to the FBW flight control system is typically either a sidestick or a yoke. The yoke is the older and more traditional device and is preferred by some pilots (even for use with FBW systems). This is because the yoke gives both pilots tactile feedback when one of the pilot moves the yoke while controlling the aircraft's pitch and roll movements, for example. That is, the pilot and co-pilot yokes are connected together such that a movement of a yoke by the pilot results in a corresponding automatic similar physical movement of the co-pilot's yoke, and vice-versa.
In contrast, sidesticks tend to be somewhat better than yokes in allowing the pilots to make relatively more rapid control inputs through simple and quick movements of the sidestick in any direction. This is because relatively less pilot manual force is required to move the sidestick as compared to the yoke. Also, the sidesticks take up much less space in the aircraft cockpit. This is because the sidesticks are relatively smaller than the yokes and are located off to the side of each of the pilot and co-pilot (i.e., left side of pilot, right side of co-pilot). A sidestick only requires one hand to grip the sidestick (e.g., left hand for the pilot, right hand for the co-pilot). In contrast, the yokes are typically located in front of each of the pilot and co-pilot and usually require both hands to operate.
However, in contrast to a yoke, when one pilot moves his/her sidestick, the sidestick of the other pilot typically does not move in correspondence. That is, the two sidesticks (i.e., one for the pilot and another for the co-pilot) are usually independent of one another in terms of issuing flight control commands. This lack of feedback to the other pilot may result in a potentially dangerous “dual input” situation where both pilots are “fighting” for control of the aircraft if both pilots are issuing aircraft flight control commands simultaneously using their own sidestick and, perhaps, unbeknownst to one another. Typically such a situation is handled by having the FBW control system algebraically sum the inputs from both pilots and with a maximum value or limit in place for each of various commandable flight control parameters which cannot be exceeded by the sum of the two inputs. Yet, it is known to couple the two sidesticks together in some implementations such that movement of one of the sidesticks results in a “feedback-type” of movement of the other sidestick.
Also, as currently implemented in most sidestick-controlled commercial aircraft, a sidestick does not provide a pilot who is issuing flight control commands with his/her sidestick with the same type of tactile (e.g., visual) feedback as does a yoke. This lack of tactile feedback somewhat deprives the pilot of a sense of how the aircraft is behaving during flight. This lack of feedback is a drawback with some pilots, particularly those pilots who have spent the majority of their careers flying aircraft with a yoke as the primary flight control input device. On its modern aircraft, Boeing still uses separate yokes for the pilot and co-pilot even with a FBW system on the aircraft. In contrast, Airbus uses a sidestick for each of the pilots on its modern aircraft with a FBW system.
In the aforementioned situation where both pilots are simultaneously issuing flight control commands using their own sidesticks (i.e., a “dual input” situation), it is known to utilize a priority scheme in which, for example, a “priority” button or switch located on the sidestick, when depressed or otherwise activated by one of the pilots and held in that position, momentarily prevents or “locks-out” any flight control command inputs from the other pilot using his/her sidestick from taking effect. The priority button or switch may also act as the autopilot disconnect or disengage switch to turn off the autopilot control in the aircraft flight control system. This activation of a priority button or switch by one of the pilots effectively deactivates the dual input situation by preventing the pilot who now does not have priority from continuing to issue flight commands using his/her sidestick. This momentary priority given to the pilot who is activating the priority button or switch on his/her sidestick usually lasts for only as long as the priority button or switch is depressed. Once the pilot releases the button or switch on his/her sidestick that pilot loses his/her priority of giving commands to the aircraft flight control system. The other pilot can then take over priority by activating the priority button on his/her sidestick.
Still further, it is known that in some implementations if one of the pilots depresses or otherwise activates the priority button on his/her sidestick for a certain period of time (e.g., 30 seconds), then that pilot obtains a “latched” priority in which that pilot can then release the button and still maintain priority. However, having to hold the priority button for 30 seconds can be a cumbersome task, particularly if that pilot is performing other functions at the same time.
Also, in this situation some type of visual and/or aural feedback is given in the cockpit to both pilots to make them aware as to which pilot currently has sidestick priority control using his/her sidestick. For example, the visual feedback regarding the current priority status may be given on the main panel of the cockpit itself (e.g., on the primary flight display), and/or also on a display located on the glareshield in front of each of the pilot and co-pilot. The aural feedback may be a recorded voice spoken in the cockpit telling both pilots who currently has priority.
Further, because the inceptor priority button mounted on the sidestick typically also functions to disconnect the autopilot function in the aircraft (e.g., traditionally on final approach of the aircraft to the runway at an airport), this combination of sidestick priority and autopilot disconnect on the same control device can have unintended consequences; for example cancelling a sidestick priority condition and allowing a failed inceptor (i.e., the priority button) to provide erroneous commands to the FBW control system in a critical phase of aircraft flight.
What is needed are aircraft sidesticks for both the pilot and co-pilot to provide input commands to an aircraft flight control system having an improved method of effectuating a “latched” (as opposed to a “temporary” or “momentary”) sidestick priority condition for either one of the pilots, which allows for a quicker and easier transition from a potentially dangerous “dual input” flight control condition to a safer flight control condition where only one pilot has sidestick priority for entering commands to the aircraft flight control system.